1. Field of the Invention
This invention relates to a biodegradable triglyceride-based base oil, to its production and to hydraulic oils containing this base oil.
2. Statement of Related Art
In the past, most hydraulic oils were produced from mineral oil. The same is still true today. In applications where the escape of oil into the environment has to be accepted as a possibility, for example through unavoidable leakages, there is an increasing demand for hydraulic oils which contain environmentally friendly ester oils, particularly those based on rapeseed oil and/or soybean oil, as their oil base. Typical applications of the type in question include the machinery and tools used in forestry, agriculture, building and the like. These applications require the use of hydraulic oils belonging to water hazard class 0-1. Ester-based hydraulic oils are capable of meeting these requirements.
However, ester oils of the above-mentioned type essential for practical application, i.e. oils based on purified rapeseed oils and/or soybean oils, more particularly those freed from aminopectins and other slime-forming substances, have two distinct weaknesses in terms of practical application, namely:
Ester oils based on polyunsaturated fatty acid systems tend to thicken rapidly, even at only moderately elevated operating temperatures, for example in the range from 50.degree. to 80.degree. C. The reason for this is the readiness of the olefinic double bonds of the ester-forming acids of the oil type in question to enter into viscosity-increasing reactions in the presence of atmospheric oxygen. Although it is known in principle that such unwanted increases in viscosity in hydraulic oils can be avoided by the addition of antioxidants, it has been found that the antioxidants hitherto preferably used in hydraulic oils based on mineral oils perform unsatisfactorily in ester oils of the type in question.
Another important limitation of hydraulic oils based on the environmentally friendly ester oils mentioned is their inadequate stability at low temperatures. For example, purified rapeseed oil has a solidification point or pour point of -16.degree. C. Even before the solidification point is reached, a remarkable increase in viscosity occurs with decreasing temperatures. The comparatively high pour point of rapeseed oil, for example in winter, poses considerable problems for the practical application of the hydraulic oils at low ambient temperatures. These problems can of course be made considerably worse in practice if, at the same time, the considerable increase in the pour point of the hydraulic oil is initiated by oxidative thickening of the above-mentioned ester oil. The addition of pour point depressants does not solve the technical problem involved. It is known that the effect of pour point depressants disappears after prolonged presence in the oil to be treated.
DE-A-39 27 155 describes an environmentally friendly base oil based on natural substances for the formulation of hydraulic oils containing a rapeseed oil and/or soybean oil as the main oil component, specially selected antioxidants and a quantity equal to the main oil component of esters of trimethylol ethane, trimethylol propane and/or neopentyl alcohol with C.sub.5-10 monocarboxylic acids or at least partly unsaturated fatty acids based on rapeseed oil, soybean oil or sunflower oil.